This invention relates to computer printers, and particularly to media transport mechanisms and methods.
It is sometimes desired to process the output of computer printers with finishing devices or finishers that perform any of a range of specialized operations (e.g. hole punching, binding, cutting, folding, stapling, collating, sorting, and job offsetting.) Different users have different needs for these specialized functions. Therefore, such finishing devices are usually sold separately from printers, and are usually standardized so as to be compatible with a common printer, or with a range of printer models or brands. This permits manufacturers to affordably produce printers and finishers in adequate quantities of each model, without low-volume custom runs of particular combinations.
Most current finishers are designed to receive the output of laser printers, which are commonly used for high volume printing applications where finishers are also required. Laser printers operate in a continuous feed manner, in that the media sheet moves through the printer at an essentially smooth rate, without stopping and starting. While there is a velocity change at the outset and conclusion of printing as the sheet is picked from a supply, and deposited in an output bin, and the velocity is not necessarily perfectly constant, the motion is essentially continuous, without any intended stopping and starting. Thus, current finishers are configured to operate at similarly continuous feed rates. This permits documents to be fed sheet-by-sheet directly from the printer to an attached finisher, with the finisher having rollers that continue to move a leading portion of the sheet, even while a trailing portion of the sheet is being printed in a process that requires the position not to be disturbed.
In comparison, ink jet printers operate in a stepped or indexed manner in which a media sheet is fed in increments, stopping and starting many times during printing. The sheet must be stationary while an ink jet print head is printing a swath of ink droplets, then rapidly indexed before the next swath is begun. Existing continuous-feed finishers are unsuitable for ink jet printers because they generate alternating tension and compression in the sheet; tension while the finisher""s rollers try to draw a sheet during printing of a swath, and compression as the rapid speed of the indexing exceeds the finisher""s intake rate, generating resistance to indexing. These effects would act to impair printing quality by losing the feed position of the sheet during printing, and by creating rubbing by feed rollers, which impairs surface finish and print quality.
A finisher may be developed specially for indexing printers, with feed provisions for matching the feed action of the printer. However, this requires more sophisticated software and hardware connections to ensure communication, and requires stepper or closed loop DC motors in the finisher to ensure precise positional control. This is not considered practical or cost effective. Moreover, it is economically impractical to develop a separate product line and inventory of finishing devices both for continuous feed (e.g. laser) printers, and indexing or step feed (e.g. ink jet) printers. And while laser printers may generate adequate market size and demand for finishers, ink jet printer types may generate limited demand for custom suited finishers below a level considered to be economically viable.
The present invention overcomes the limitations of the prior art by providing a printing apparatus with an index-feeding printing system having a first media path, a continuous feed finishing apparatus having a second media path, and a media buffer device having a third media path connecting between the first media path and the second media path. The third path may have a first portion communicating with the printing system, and a second portion communicating with the finishing apparatus, with the first and second portions merging with each other to form a common path portion terminating at a common path dead end, such that a media sheet from the printing system stops at the dead end and reverses direction for feeding to the finishing apparatus.